1. Field of the Invention
The invention relates to an MOS capacitor with improved frequency characteristics. The invention also relates to a Vpp switch circuit and charge pump circuit employing such an MOS capacitor, and an EEPROM, a microcomupter and an IC card.
2. Description of the Related Art
FIGS. 14 and 15 each illustrate a conventional MOS capacitor. In this capacitor, gate oxide film 8 is disposed between a polysilicon film 2 and a P substrate 7 for forming polysilicon film 2 spaced from P substrate 7. An oxide film 9 is disposed on the polysilicon film 2. Aluminum electrode layers 3 and 5 are disposed on oxide film 9. A portion of aluminum electrode layer 3 passes through oxide film 9 and contacts polysilicon film 2 to form aluminum contact 6. On the other hand, a portion of aluminum electrode layer 5 passes through oxide film 9 and contacts N diffusion region 10 in the P substrate 7 to form a field contact 4. This field contact 4 is disposed along a side 1a (see FIG. 16) of a rectangular field 1, region of P substrate 7 spaced from a field oxide film 11.
Application of a voltage between aluminum electrode layers 3 and 5 having a value greater than the threshold voltage value Vth required for channel formation forms an N inversion layer 12, acting as a channel, progressively from the portion adjacent to N diffusion layer 10 to within field 1. Charges accumulate at the portion where field 1 is opposite the polysilicon film 2 with the gate oxide film 8 therebetween. This portion functions as a capacitor. That is, as shown in the equivalent circuit of FIG. 16, a plurality of small capacitors are connected by resistors therebetween toward a side 1b opposite side 1a of field 1. Applying a voltage V causes voltage to be applied starting from the small capacitor adjacent to side 1a of field 1 and progressing successively to the other capacitors toward side 1b. This means that more time is required for the charging and discharging of the small capacitors closer to side 1b opposite side 1a than those closer to side 1a of field 1. For this reason, when voltage changes occur at high frequencies, the capacitors are incapable of functioning as desired.
In manufacturing an MOS capacitor, subjecting the surface of P substrate 7 where the channel is formed to capacitor doping allows the threshold voltage value Vth required for channel formation to be reduced. Reducing the threshold voltage value Vth in this way reduces the charging and discharging time required for even the small capacitors closer to the side 1b of field 1 opposite side 1a. Therefore, an MOS capacitor capable of operating at high frequencies can be formed.
Using such an MOS capacitor having a low threshold voltage value Vth in a Vpp switch circuit used, for example, to select either "supply voltage Vpp for writing into EEPROM" or "not supply voltage Vpp for writing into EEPROM", causes more leakage current to flow, which often results in malfunctioning of the capacitor. Even when a charge pump circuit which develops a voltage Vpp for writing into an EEPROM or the aforementioned Vpp switch circuit is not operating, the current consumption becomes large due to a large load capacitance because of the capacitor built into the circuit.
As described above, conventional MOS capacitors have the problem of poorer operation at higher frequencies. Another problem is that decreasing the threshold voltage value Vth to allow the capacitor to operate at high frequencies often results in malfunctioning when used in a Vpp switch circuit. A further problem is that Vpp switch circuits and charge pump circuits using conventional capacitors have a higher current consumption due to higher load capacity.